Charge transfer and X-ray absorption investigations in aluminium and copper co-doped zinc oxide nanostructure for perovskite solar cell electrodes

This study explores influence of charge transfer and X-ray absorption characteristics in aluminum (Al) and copper (Cu) co-doped zinc oxide (ZnO) nanostructures for perovskite solar cell electrodes. Sol-gel technique was employed to synthesize the nanostructures, and their optical and morphological properties were investigated. X-ray diffraction (XRD) analysis confirmed high crystallinity and also single-phase composition of all the samples, particularly up to 5% Al co-doping. Field emission scanning electron microscopy (FESEM) exhibited the formation of pseudo-hexagonal wurtzite nanostructure and the transition to nanorods at 5% Al co-doping. Diffuse reflectance spectroscopy indicated a reduction in the optical band gap of co-doped zinc oxide from 3.11 to 2.9 eV with increasing Al doping. Photoluminescence spectra (PL) exhibited a decrease in peak intensity, suggesting enhanced conductivity in ZnO, also confirmed from I-V measurements. Near-edge X-ray absorption fine structure (NEXAFS) analysis depicts that charge transfer from Al to oxygen (O) species enhanced the photosensing properties of the nanostructure, which was supported by FESEM micrographs and PL spectra. Furthermore, the study discovered that 5% Al co-doping significantly reduced the density of emission defects (deep-level) in Cu–ZnO nanostructure. These findings highlight the potential of Cu and Al co-doped ZnO materials for perovskite solar cell electrodes, as their improved optical and morphological properties resulting from charge transfer could enhance device performance. The investigation of charge transfer and X-ray absorption characteristics provides valuable insights into the underlying mechanisms and behaviors of the co-doped ZnO nanostructures. However, further research is required to delve into the intricate hybridization resulting from charge transfer and explore the broader impact of co-doping on other properties of the nanostructures, enabling a comprehensive understanding of their potential applications in perovskite solar cells.


Materials and methods
The pristine ZnO and Al x Cu y Zn 1−x−y O (y=0.00, 0.005, x=0.00, 0.005, 0.01, 0.03, and 0.05) samples were prepared using sol-gel approach. Zinc acetate dihydrate and copper acetate monohydrate, along with aluminum chloride hexahydrate, served as the source of zinc and dopants, respectively. All chemical reagents, including copper acetate monohydrate, sodium hydroxide (NaOH), aluminum chloride hexahydrate, and zinc acetate dihydrate, were obtained from Sigma-Aldrich.
In the typical process, the aforementioned reagents (excluding NaOH) are dissolved in distilled water according to their stoichiometry. NaOH solution is dissolved in de-ionized water and it is added to solution dropwise with constant stirring by using temperature of 60 • C, maintaining a pH of approximately 9-10, until milky precipitates formed. The obtained precipitates were centrifuged at 6000 rpm for 2 minutes, then washed/rinsed using de-ionized water/ethanol. Subsequently, they were dried for 7 hours in an oven at 100 • C and then annealed at 400 • C by using muffle furnace for 12 hours. Finally, the samples were ground to a fine powder by utilizing a mortar and pestle. Using a similar method, samples with different concentrations of copper and aluminum were prepared. The synthesized samples were ZnO The crystalline behavior of the synthesized samples was confirmed using X-ray diffraction (PANalytical Model-X'pert pro, Netherlands) with Cu K α radiation ( =0.154 nm). The diffraction patterns were recorded www.nature.com/scientificreports/ from 20 • to 80 • Bragg's angle with a step size of 1 • per minute to obtain fine structural resolution so that it can be used for Reitveld's refinement. Morphological and structural studies were conducted using FE-SEM images captured by a Hitachi (Japan) Model-SU 8010 series (resolution of 1 nm) and a landing voltage of about 1 KV. The band gaps of different samples were calculated using UV-DRS spectroscopy, and PL analysis was performed to analyze the fluorescence nature of the synthesized samples. The I-V characteristics for photo-sensing investigations were performed using the electrochemical workstation M204 (Metrohm AutoLab) at the central facility lab, Dr. SSB UICET, Panjab University, Chandigarh. Near-edge X-ray absorption fine-structure (NEXAFS) evaluation for O K-edge and Zn L-edge was carried out on the samples at the 10D (XAS-KIST) beamline of the Pohang Accelerator Laboratory (KIST-PAL beamline) in South Korea. Cu K-edge NEXAFS data were collected in transmission mode at BL8, Synchrotron Light Research Institute (SLRI), Thailand.
Results and discussion X-ray diffraction study. The X-ray diffraction approach was used to investigate structure of materials, determine the crystallinity of the samples, nanoparticle size, phase purity, and lattice parameters. The polycrystalline peaks observed in Fig. 1a match to the Miller Indices (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of pure wurtzite hexagonal structure. Among these peaks, the (101) plane (2θ = 36.3 • ) exhibits the highest intensity peak, indicating a highly crystalline and well-defined nature. The absence of impurity peaks in all samples (pH ∼ 9-10) confirms the complete decomposition of all precursor solutions, which matches with previous reports 34,35 . The peaks corresponding to all the samples closely match with the JCPDS data (01-089-0510,01-079-0206). Analysis of the (101) plane peak positions for pristine ZnO and Al(3) reveals a shift to higher 2 θ values compared to other co-doped samples (Fig. 1b). This right peak shift confirms the substitution of copper and aluminum ions into the ZnO matrix 36 . The shifting of peaks occurred due to smaller ionic radii of Al 3+ and Cu 2+ comparing to Zn 2+ ion 37 .
The line broadening observed in the main (101) peak of pristine and co-doped ZnO materials indicates the nanometer range of the synthesized samples. The crystallite size (D) of the prepared powder samples were determined utilizing the Scherrer equation, which uses line broadening of the (101) peak 38 . The Scherrer formula is given by: D = 0.9 /β cos θ , where D depicts crystallite size, is referred as the wavelength of the X-ray (1.5404 Å), β represents full width at half maximum (FWHM) of XRD peak, and θ denotes angle of diffraction peak.
The samples prepared by sol-gel method exhibit a range of crystallite size between 43 and 90 nm, as summarized in Table T1 (supplementary text). The results reveal that the incorporation of copper and aluminum dopants (Cu(0.5) and Al(0.5)) leads to increment in crystallite size when compared with pristine sample. In particular, crystallite size of Cu(0.5) is larger when compared to Al(0.5). Furthermore, both Cu(0.5) and Al(3) exhibit larger crystallite sizes compared to the pristine and other co-doped samples, as observed in Fig. 1c.
The crystal structure investigation has performed utilizing Rietveld refinement of pristine and co-doped samples of ZnO XRD data 39 . The Rietveld refined XRD patterns by considering the P63mc space group for the pristine ZnO hexagonal structure, are revealed in Fig. 2, confirming presence of ZnO wurtzite structure. The other Rietveld refineed patterns of co-doped samples compared with pristine ZnO is shown in Fig. S1 (supplementary text). The calculated structural parameters of synthesized samples are recorded in Table T1 (supplementary text), demonstrating fitting quality of the experimental data. The non-variant c/a ratio (1.602) is less than the ideal value of 1.633, indicating an increasing behavior of the u-parameter (bond length parallel to c-axis), while keeping the tetrahedral distance uniform through deformation of tetrahedral angles, considering long-range interactions 40,41 . With the modification of the c/a ratio, wurtzite structure changes from ideal arrangement in real Strain, stress and energy density analysis. To analyze stress, strain, and energy density, originating due to the crystal structure energy (Gibbs free energy), Williamson and Hall (W-H) plots were investigated using the variation in the plane parameters as per the equations (Eq. 1-Eq. 3). W-H utilized convolution to explore the impact of size and strain parameters, resulting in broadening of peaks. The broadening of the peak caused by crystal size β (determined by the Scherrer formula) is dependent on the angle, following a variation of 1/ cos θ , while the broadening due to strain (represented as β e = 4ε tan θ ) varies as tan θ . Therefore, the W-H equation is: Equation (1) is referred to as Uniform Deformation Model (UDM), assuming the value of strain within the material is uniform in every direction, considering crystal to be isotropic. Similarly, the Uniform Stress Deformation Model (USDM) is construct on certain assumptions. Hence, Hooke's relation, which asserts a linear proportionality between strain and stress ( σ = Y ε , here Y is Young's modulus) is employed for the calculation of stress. Consequently, the W-H equation (Eq. 1) is refined as follows: (1) β hkl cos θ = k D + 4ε sin θ  Linear regression is performed to get W-H plots, as depicted in Figs. S11 to S16 (supplementary text), where the slope yielded insights into energy density, stress, and strain, while intercept is utilized for the estimation of crystal size 39 . Within aforementioned models, lattice strain, lattice stress, and energy density are calculated with certain approximations. Remarkably, the crystallite size acquired utilizing these models exhibited a strong agreement with determined average size through Rietveld refinement.  Figure 5a,b shows the histogram for average diameter of Cu(0.5) nanoparticles, and average length of Al(5) nanorods, respectively. The nanostructures exhibit high density and uniformity, likely attributed to the growth mechanism. Figure 3a,b depict the formation of well-distributed pseudo-hexagonal-shaped copper-doped ZnO nanostructures (Cu(0.5)), consistent with the previous observations 39 . A magnified view of the pseudo-hexagonalshaped (Cu(0.5)) nanostructure is shown in Fig. 3c. The substitution of aluminum doping leads to nanoparticle agglomeration, as evident in Fig. 3d,e. The agglomeration of pseudo-hexagonal nanoburgers (enlarged view of Fig. 3e) is observed in Fig. 3f.
The FE-SEM images in Fig. 4a demonstrate the formation of well-dispersed pyramid-like nanostructures (Al (1)). These nanopyramid-like assemblies consist of dozens of pseudo-hexagonal rod-like nanostructures, acting as building block units (Fig. 4b). With increased aluminum doping concentration, uniformly distributed micro-flowers composed of nanorods (Al(5)) are formed, as depicted in Fig. 4d. The cauliflower shape of Aldoped ZnO nanostructure is highlighted by the marked circle in Fig. 4e. Similar FE-SEM images of nanoflowers and nanorods are described in previous reports 44,45 . The flower-like ZnO nanostructures appear to form due to the branching of central rods rather than the aggregation of rod-like nanostructures. The magnified image of The growth mechanism of nanostructures of ZnO can be understood by considering the nucleation and crystal growth processes. It is hypothesized that the growth mechanism is governed by a nucleation/crystal  www.nature.com/scientificreports/ growth approach, wherein nucleation process arise initially, following crystal growth process. The nucleation process is relatively slow compared to the faster crystal growth process, especially at higher pH values. During the nucleation process, the [Zn(OH) 4 ] 2− growth component readily attaches to surface of ZnO seed, inducing the growth of seed nuclei along the c-axis and the generation of cauliflower-shaped nanostructures. With the increase of the reaction time and annealing temperature, re-nucleation of side branches occurs, resulting in the growth of nanorods. The mechanism underlying nucleation and growth processes involved in this preparation can be elucidated through Fig. 6, which provides further insights into morphological modification of the ZnO nanostructures.
The elemental percentage calculated from Energy Dispersive X-ray (EDX) microanalysis as shown from Figs. S3, S4, S5, and S6 (supplementary text) are tabulated in the isnets of these figures.
Diffuse reflectance spectroscopy (DRS) analysis. The absorption spectrum of pristine and various co-doped zinc oxide were recorded in range of 200-800 nm. The absorption edges of all the pristine and codoped samples were found to be similar, indicating a small difference in energy bandgap (Fig. 7).
Diffuse Reflectance Spectroscopy (DRS) techniques are utilized for the study of band gap. The band gap energy in semiconductors represents energy needed to excite an electron toward the conduction band. The band gap energy of pristine and co-doped zinc oxide is calculated utilizing the Tauc, Davis, and Mott relation 48 for indirect band-gap: Here, α , ν , E g , and A represent absorption coefficient, frequency of light, band gap energy, and proportionality constant, respectively.
The band gaps are evaluated by plotting ( αhν) 1/2 against Energy (E) (Fig. 7). The band gap values are calculated by extrapolating straight line to the intercept axis ( Fig. 7)a,b,c,d,e,f and g. The calculated band gap values ranged from 3.11 to 2.9 eV, consistent with previous reported values of 3.1-3.2 eV by Mott et al. 49 .
In the present study, band gap of co-doped samples reduced due to variations in dopant size, host material, and annealing effects, leading to defect formation. The absorption edge shifted toward longer side of wavelength, indicating the formation of extra energy levels within band gap of zinc oxide with the incorporation of Al dopants into Cu-ZnO matrix. Therefore, less energy is needed for the excitation of electrons into the conduction band, resulting in increased charge transfer. The decrease in the band gap of zinc oxide with increment in Al dopant concentration can be attributed to increase in grain size and decrease in carrier concentration 50 (Fig. 7h), consistent with the analysis of XRD showing an increase in crystallite size.
Analysis of photoluminescence spectra (PL). The pristine ZnO sample exhibits a large intensity ratio of visible peak/ultraviolet, indicating fluorescence. Photoluminescence (PL) analysis was conducted to investigate optical properties of co-doped ZnO nanostructures (Fig. S10 in supplementary text). The peak analysis of PL for pristine and co-doped samples is presented in Fig. 8. The different peak positions (Zn1, Zn2, Zn3, Zn4, Zn5) and their corresponding values of full-width at half-maximum (FWHM) are shown in Table T2 (supplementary  www.nature.com/scientificreports/ the case of Al(5) samples, all peaks show a decrease in wavelength compared to pristine ZnO, indicating reduction in recombination rate of free excitons. The decrease in intensity of the second peak (Zn2) with varying doping amounts may be because of the net increment in defects, like zinc or oxygen vacancies 51,52 , which can be correlated with the O K-edge NEXAFS analysis revealing existence of oxygen vacancies, discussed in next sections. The photoluminescence spectra of ZnO and (Al,Cu) co-doped zinc oxide show the presence of two peaks. The strong emission peak nearby 366 nm (ultraviolet region) is ascribed to increased number of charge carrier recombination rates in the near-band edge (NBE). A lower number of free electrons in the system leads to a decrease in conductivity and, consequently, a decrease in photosensing in photodetectors. The broad emission peak observed in green band is ascribed to the creation of oxygen vacancies 53,54 . Photoluminescence (PL) serves as an optical approach to identify defects in a sample. The UV peak arises from the fast recombination of charge carriers. The decreased band gap observed in co-doped ZnO nanostructure, compared to pristine zinc oxide (3.37 eV), suggesting the presence of significant defect levels in prepared zinc oxide sample. The narrowing in the band gap is ascribed to donor impurities creating energy levels nearby conduction band. Therefore, when the PL intensity is weak, the recombination rate of charge carriers (photo-generated) slows down, leading to an increase in conductivity and enhanced photosensing in photodetectors 53,54 . The PL intensity peaks of Al-doped and (Cu,Al) co-doped zinc oxide (Fig. 8c,d,e and f) are lower compared to PL spectra of ZnO and Cu-doped ZnO (Fig. 8a,b), indicating an improvement in the optical properties of co-doped zinc oxide. This can be ascribed to the presence of doped ions creating numerous electron traps, suppressing the charge carrier recombination rate and promoting charge transfer in the ZnO system. Comparing all the PL intensities, it is evident that the co-doped zinc oxide samples (Fig. 8c,d,e and f) exhibit lower recombination rates and thus exhibit better optical properties 51,52 .
In the zinc oxide lattice, the structure is hexagonal close-packed (HCP) with Zn 2+ ions surrounded by six O 2− ions. The introduction of aluminium and copper ions into the zinc oxide lattice can modify the hybridization state of the surrounding atoms and enhance optical and electronic properties of the material ZnO 55 . The hybridization state of aluminium and copper ions depends on their coordination environment and the specific doping conditions. The hybridization state of copper ions can range from sp 2 to sp 3 d 2 depending on their coordination Near edge X-ray absorption fine structure(NEXAFS) analysis. Near-edge X-ray absorption fine structure (NEXAFS) is a aggressive approach employed to explore various aspects of electronic structure, such as hybridization, crystal field strength, chemical valency of cations, and assess the presence of sample vacancies. O K-edge NEXAFS. To understand co-doping effect, NEXAFS spectra of Al x Cu y Zn 1−x−y O (y=0.00, 0.005, x=0.00, 0.005, 0.01, 0.03, and 0.05) at O K-edge are described from Fig. 9. The O K-edge represents the 1s core state transition to empty derived states of oxygen (2p), hybridized with small 3d and wide 4sp bands [61][62][63] . From  Fig. 9a, O K-edge spectra (normalized) of Cu(0.5), Al(0.5), CuAl(0.5), and Al(1) resemble that of pristine ZnO. However, the spectra of Al(3) and Al(5) are different from pristine ZnO, suggesting a modified nanostructure. These spectra exhibit a normalized behavior with the same area in the range of 550 and 570 eV. In Fig. 9a, four peaks (A 1 , A 2 , A 3 , and A 4 ) are recognized in this spectra, centered at 535, 537, 539, and 542 eV, respectively. The distinctive spectral feature between the energy range of 535-538 eV (denoted by A 1 and A 2 ) can be ascribed to hybridization of O 2p state with Zn, Cu, Al 3d/4s states, the region in between 538 and 542 eV (A 3 and A 4 ) corresponds to hybridization of O 2p state with Zn, Cu, Al 3p states. By comparing the spectrum of Al(3) and Al (5) with pristine ZnO and other doped samples, two additional spectral features at 538 eV (A 3 ) and 542 eV (A 4 ) are observed. The intensity of these features decreases and increases with Al doping, respectively, indicating a modification of the local electronic structure of these Al doped Cu-ZnO nanostructures 64 . Therefore, this new feature at 538 eV, nearby the minimum of conduction band, may be assigned to result of dopant 3d hybridized with O 2p state. The broadening of spectral features observed in higher Al co-doped zinc oxide at 538 eV (A 3 ) and 542 eV (marked by A 4 ) ascribed to the appearance of oxygen vacancies or morphology effect observed in FESEM micrographs. The presence of new spectral features and increment in broadening imply that Al atoms fill up interstitial sites 65 in the zinc oxide surrounding oxygen vacancies. Consequently, because of the presence of oxygen vacancy, more charge can be transferred from the doped Al ions, resulting in Al ions tetrahedrally coordinated to oxygen sites in the ZnO matrix 66 . The broadening of these spectral features in Al (5) is higher compared to the other samples, indicating high assembly of oxygen-related defects and magnetic moment, ascribed to the appearance of oxygen vacancy and morphological effect 67,68 . Figure 9b shows the difference spectra illustrating the variation with Al-Cu co-doping.
Chen et al. 64,69 provide an explanation for the spectral variation observed, which ascribed with the concept of charge transfer. As depicted from Fig. 9, intensity of highly Al co-doped ZnO is observed to decrease compared to pristine ZnO. This reduction can be attributed to a higher occupation degree of the O 2p state in zinc oxide, which is explained from the concept of enhanced charge transfer from Al (adsorbed) to O 64 . The presence of multiple electron traps and oxygen vacancies can be correlated with photoluminescence (PL) and Zn L-edge NEXAFS spectra. The hybridization between the orbitals of 3p incorporated Al and orbitals of 2p adjacent to O promotes charge transfer from Al to O 64 , which is also explained in Fig. 12. This change in spectral shape of doped ZnO indicates the occurrence of significant morphological changes in the material.
It has suggested that Al can either be substituted into the lattice of ZnO system by incorporating for Zn ions or create a layer onto the grain boundaries or surface of ZnO powder 64 . In the XRD pattern, no generation of secondary phase is observed; however, possibility of Al 2 O 3 phase cannot be ignored 70 . In the case of pristine ZnO, the A 3 peak is clearly visible, but in heavily Al-doped ZnO, the   Figure 10 illustrates the normalized Zn L 3,2 -edge of Cu-incorporated zinc oxide. The L 3 and L 2 regions correspond to the Zn (2p) to Zn (4s) and Zn (3d) antibonding state, respectively, following Mott selection rules 71 . The spectral features in the L 3 region represent electron transition from Zn (2p) to Zn (3d) state. The pre-edge feature, indicated by a downward arrow at C1, is influenced by the Zn 4s band transition. In contrast, the spectral feature in the L 2 region results from multiple overlapping bands 72 , exhibiting symmetrical position and shape. This also suggests, Zn defect should not contribute to magnetic properties of copper-incorporated zinc oxide. The Zn L-edge spectrum in ZnO generally arises from core shell electrons. The degenerate states 2p 3/2 and 2 p 1/2 emerge due to spin-orbit coupling, leading to multiplets centered at 1026 and 1034 eV. The crystal field (octahedral) raise degeneracy of 2 p 3/2 and 2 p 1/2 levels, resulting in the creation of t 2g and e g sub-band symmetries 73 . The fine structure multiplet arises from two effects: (1) interaction in-between the 3d (electron) and 2p core hole, and (2) crystal field created by neighboring ions at a Zn 2+ site 74 .
When investigating Al co-doping in the Cu-ZnO system, it is observed that up to 1 at.% of Al does not alter the degenerate levels. However, for Al doping above 1 at.%, the splitting of levels becomes evident, as indicated by C 1 (t 2g )-C 2 (e g ). The crystal field splitting results in sharp peaks due to the separation of degenerate levels. The peak of the Al(5) sample is shifted in energy due to an increasing nature of the crystal field parameter 10Dq (marked by C 2 ), which is consistent with the results from XRD, PL, FESEM, and O K-edge analysis ( Fig. S9 in supplementary text).
To determine 10Dq in Fig. 10, we examine the sharp pre-edge feature that quickly varies with the change in 10Dq. The shifted peak of the Al(5) sample is also observed in the pre-edge feature, marked by C 1 , while another feature marked by C 2 is also present. Additionally, there is a shifted peak of Al(3) marked as ' A' . These shifted peaks can be sensitive to morphology and can be correlated with FESEM micrographs. Therefore, by considering all the results, it appears that heavy doping of Al in Cu-ZnO (Al(5)) exhibits a significant difference in peak position compared to pristine and other co-doped ZnO samples. Moreover, the density of unoccupied states of Zn increases, suggesting the occurrence of charge transfer.
The crystal field splitting is examined through the energy difference between the orbitals t 2g and e g , and this separation is referred to as 10Dq (the energy of crystal field splitting). The value of 10Dq can be determined from the absorption spectrum of the complex. The t 2g orbitals are 4Dq below the average energy level, while the e g orbitals are 6Dq above the average energy level. By using the equation E = h c/ (where represents the wavelength of the absorbing radiation, h denotes Planck's constant, and c represents velocity of light), value of 10Dq can be determined. When the complex absorbs light with the appropriate wavelength, the electron is shifted from ground state (t 2g orbital) to the excited state (e g orbital). The splitting of d orbitals in an octahedral crystal field is illustrated in Fig. S7 (supplementary text). Zn L-edge measurements revealing splitting of e g and t 2g sub-bands by increasing the amount of Al doping. This is explained by considering the orbital hybridization in-between (Zn 2+ ) d and 2p (O atom) orbitals, orbital concepts, crystal field, and band anti-crossing (BAC) interaction 75,76 . The transition from bonding to antibonding states, induced by the BAC interaction of the absorbing electron, accounts for the splitting of e g and t 2g sub-bands. The magnitude of 10Dq is influenced by several factors, such as the geometry of complexes, charge on metal ions, nature of ligands, and the position of the metal in the 1st, 2nd, or 3rd transition series. As a result, the degenerate set of d orbitals splits into two sets: e g orbitals with higher energy, including d x 2 −y 2 and d z 2 , and t 2g orbitals with lower energy, including d xy , d yz , and d zx orbitals.
Cu K-edge NEXAFS. The Cu K-edge NEXAFS of Al co-doped Cu-ZnO nanostructures and appearance of Cu +2 state in Cu are illustrated in Fig. 11. The presence of a pre-edge hump at an energy of approximately 8997 www.nature.com/scientificreports/ eV (marked by downarrow) indicates a change in symmetry, which is further confirmed by the formation of nanorods observed in FESEM micrographs at higher Al doping levels.
The intensity of main peak edge also increases after Al co-doping, ascribed to the concept of charge transfer. As the Al doping level increases, the density of unoccupied states increases, and more charge transfer occurs, as evidenced by the analysis of the Zn L-edge. The Cu 3 d 9 4 s 0 orbital can facilitate the transfer of electrons from Al to Cu/Zn, considering that Al 3+ occupies the Zn 2+ site in the Zn matrix. Consequently, changes in the HOMO intensity 33 and the transfer of charge from HOMO to LUMO occur. The phenomenon of charge transfer has also been discussed by Chen et al. regarding ferromagnetic properties 64,69 , as well as by Borgwardt et al. in the context of charge transfer dynamics following the emergence of interfacial electronic states at Dye-Sensitized zinc oxide interfaces 77 . The hybridization of the 3sp orbitals of incorporated Al with the 2p orbitals of O atoms promotes the transfer of charge from Al atoms to O atoms (Fig. 12). This concept can also be correlated with the results of O K-edge NEXAFS analysis. Photo sensitivity. Figure 13 demonstrates the photoilluminated current of aluminium and copper codoped zinc oxide nanostructures through I-V measurements. Using bias voltage of 1.5 V, the currents (photoilluminated) of pristine zinc oxide, Al(0.5), and Al(5) are observed as 0.5 µ A, 0.7 µ A, and 1.4 µ A, respectively. The low current values indicate the appearance of good crystalline quality of nanostructures. Furthermore, current value linearly enhances with the applied voltage, demonstrating a good ohmic contact in samples. The resistance values of pristine zinc oxide, Al(0.5), and Al(5) co-doped zinc oxide are calculated as 3 M , 2 M , and 1 M , respectively. The zinc oxide nanostructure reveals n-type semiconductor behavior because of the presence Figure 11. Normalized Cu K-edge NEXAFS spectra for Al x Cu 0.5% ZnO (x=0.0, 0.5%, 3%, and 5%) showing the variation with Al-doping in Cu(0.5%)-ZnO system. www.nature.com/scientificreports/ of defects like zinc interstitial and oxygen vacancies 78 . With the increment of Al doping, the number of carrier concentration also increases, as Al carries one excess valence electron compared to Zn. The decrease in resistivity with up to 5% Al co-doping is attributed to the appearance of an free electron (extra) in the conduction band, originates with the doping of Al 3+ at Zn 2+ sites 79 (Fig. 13b). Hence, after comparing the currents (photoilluminated), it can be observed that the aluminium and copper co-doped zinc oxide nanostructure exhibits improved photosensing capabilities.

Conclusion.
This research provides valuable insights into effects of Al co-doping on the electrical, optical, and structural properties of ZnO nanostructures. The substitution of Al ions leads to the emergence of (Cu,Al) co-doped zinc oxide samples with improved crystallinity, reduced band gap, and enhanced sensing capabilities. The increase in grain size, reduction of visible emission defects, and enhancement of O/Zn stoichiometry contribute to the enhanced photosensing performance in photodetectors. The PL and NEXAFS spectroscopy analyses confirm the appearance of charge transfer processes and creation of electron traps, further improving optical properties of the co-doped ZnO, and hence the improved photosensing. This study highlights potential of Al doping as a viable strategy for advancing the performance of photodetectors/solar cell electrodes, offering a promising avenue for future applications that require transparent, durable, cost-effective, and portable sensing devices.

Data availability
The crystallographic data generated is available in the Supplementary file. The other data may be provided on request from corresponding author Sanjeev Gautam (sgautam@pu.ac.in).